This invention relates to divalent lanthanide metal complexes, to processes for their preparation and to light emitting devices containing the complexes.
Flat panel displays are the critical enabling technology for many current applications, including laptop computers and xe2x80x9chead upxe2x80x9d displays, as they offer several potential advantages over conventional cathode ray tube displays, including compactness and low power consumption.
Currently, the flat panel display market is dominated by liquid crystal technology, but these materials suffer several drawbacks including small operational viewing angles, poor image contrast and high power consumption. As an alternative technology for flat panel displays, electroluminescent (EL) displays using semiconducting organic polymers offer the potential of lower cost, improved viewing angles, better contrast and lower power consumption. However, these materials often have broad emission profiles, resulting in poor chromaticity and reduced device efficiency.
Typically, a flat panel device is a multilayer assembly of structurally important films consisting of a transparent electrode, insulation, phosphor and metal electrode. All are important materials in device fabrication, but the single most important element in the development of a multi-colour electroluminescent device is the phosphor.
It is known that organometallic complexes can be used as phosphors in electroluminescent devices. For example, U.S. Pat. No. 5,552,547 describes complexes of aluminium, gallium and indium in which one of the ligands acts as a xe2x80x9cbuilt-inxe2x80x9d fluorescent dye. The colour of the light which is emitted from the complex is determined by the ligand which acts as the dye.
Lanthanide-based materials are gaining popularity as phosphors for thin film devices, as they offer several potential advantages over other light emitting species: such as narrow emission linewidths, the potential for device structures with efficiencies greater than 25% and excellent Commission Internationale de I""Eclairage (CIE) colourmap coordinates. To date, two distinct types of lanthanide metal based phosphors have been reported in the literature; those based upon solid-state inorganic matrices doped with small amounts of the lanthanide ion and molecular coordination complexes. Conventional inorganic thin film EL devices are based upon solid-state phosphors and this group of materials are amongst the most extensively studied of all EL devices in the literature. Unfortunately, these solid-state devices, often based on doped II-VI materials, require large driving voltages and this has hampered their development in portable thin film displays, although there are reports of adequately functioning thin film structures. Recently, several groups of workers have recognised the potential of molecular organometallic phosphors to incorporate the processing and manufacturing advantages of organic materials with the emissive properties of the solid-state materials and reports of the use of lanthanide coordination complexes as hybrid materials are becoming increasingly common. To date these devices have been based almost exclusively upon trivalent europium (red emitting) and terbium (green emitting) complexes with bidentate oxygen donor ligands such as benzoylbenzoate and acetylacetonate derivatives.
M. A. Pavier et al., Thin Solid Films, 284-285 (1996) 644-647, describe electroluminescence from dysprosium- and neodymium-containing Langmuir-Blodgett films. The metal is in the trivalent state in the complexes and the ligand used is a pyrazolone-based molecule in which the binding to the metal by the ligand occurs via a beta-diketonate-type arrangement.
The trivalent europium complex with phenanthroline and thenoyltrifluoroacetone is disclosed in Sano et al, Jpn. J. Appl. Phys., vol.34 (1995), p. 1883-1887 and Campos et al in J. Appl. Phys., vol.80, no.12 (1996), p. 7144-7150. Both Sano et al and Campos et al teach the use of the complexes to provide red light in electroluminescent devices. Like the complexes disclosed by Pavier et al, it is the beta-diketonate part of the ligand which binds to the metal.
The synthesis and structure of bis(tris(3,5-dimethtylpyrazolyl)borate)samarium (II) is described by Takats et al in Organometallics 1993, 12, 4286-4288. However, there is no mention of the light emitting properties of the compound, only its structure and that of its reaction product with azobenzene. The syntheses and structures of the compounds bis[hydrotris(3-tert.butyl-5-methylpyrazolyl) borato]samarium and bis[hydrotris(3-tert.butyl-5-methylpyrazolyl) borato]ytterbium are described by Zhang et al. in New J. Chem. 1995. 19. 573-585. Certain bis- and mono-hydrotris(pyrazolyl)borate complexes of samarium and ytterbium are also described by J. Takats, J. Alloys and Compounds 249 (1997) 52-55.
Light emitting devices containing lanthanide (III) complexes are described in WO 98/06242. Trispyrazolylborate complexes of non-lanthanide metals are also mentioned but only as electron transporting hosts for phosphors. There is no mention of the use of trispyrazolylborate complexes as phosphors, of complexes of these ligands with lanthanide metal ions or of lanthanide (II) complexes at all.
One of the problems associated with the optical emissions from the trivalent lanthanide metal ions is that they are dominated by the relatively weak spin forbidden fxe2x80x94f transitions. Also, the wavelength of the emission from these transitions is generally independent of the ligand in the complex and is therefore difficult to tune by altering the nature of the ligand. The present invention solves these problems by providing a new class of lanthanide metal complexes for use as phosphors in light emitting devices in which the lanthanide metal is in the +2 oxidation state i.e., it is divalent.
Accordingly, the present invention provides a light emitting device comprising a complex containing a divalent lanthanide metal cation complexed with from one to three polydentate ligands.
Unlike the emissions from the trivalent lanthanide metal ions, emissions from the metal in the divalent state may arise from both inter-shell transitions between the 4f65d1 excited state and the 4f7 ground state and charge transfer (CT) transitions, both of which are quantum mechanically allowed and therefore potentially very efficient. Furthermore, unlike the fxe2x80x94f transition of trivalent species, the wavelength of the emission is ligand-dependent and, therefore, potentially tunable. Emission linewidths are also broadened since the transition is affected by differences in metal-ligand bond lengths between the ground and excited states of the molecule.
Preferably, each ligand in the complexes of the invention comprises one or more pyrazolyl groups, optionally substituted and optionally fused with a substituted or unsubstituted, heterocyclic or carbocyclic, aromatic or non-aromatic, ring system, and one of the nitrogen atoms of the pyrazolyl groups forms a coordinate bond to the metal. More preferably, each ligand is a trispyrazolylborate anion, the pyrazolyl groups each being optionally substituted and optionally fused with a substituted or unsubstituted, heterocyclic or carbocyclic, aromatic or non-aromatic, ring system, optionally substituted at the boron atom.
Suitable complexes for use in the invention are desirably those having the formula (I):
[(Z(L)3)pM]Aqxe2x80x83xe2x80x83(I)
wherein
Z is a carbon atom or R1xe2x80x94B fragment
p is 1 or 2
q is 2-p
A is a counterion
R1 is: (i) hydrogen, aryl or aralkyl each optionally substituted with from one to five halogen or C1 to C6 alkyl groups; or (ii) C1 to C6 alkyl, C2 to C6 alkenyl or C2 to C6 alkynyl each optionally substituted with one or more halogen atoms
each L is covalently bound to Z and is independently selected from a group of the formula (II) or (III) 
in which R2, R3 and R4 are independently selected from: (i) halogen, cyano, nitro, sulphono, amino, C1 to C6 alkylamino, C1 to C6 alkylamido, carboxyl, C1 to C6 alkyloxycarbonyl, hydroxy, C1 to C6 alkoxy, C1 to C6 alkylcarbonyloxy, C1 to C6 alkylcarbonyl C1 to C6 haloalkoxy and hydrogen; (ii) aryl or aralkyl each optionally substituted on the aryl ring or, for aralkyl, on the alkylene chain with from one or more of the groups mentioned under (i) above; and (iii) C1 to C6 alkyl, C2 to C6 alkenyl or C2 to C6 alkynyl each optionally substituted with one or more of the groups mentioned under (i) and (ii) above
or either R2 and R3 or R3 and R4 are linked so as to form a fused, aromatic or non-aromatic, ring system with the pyrazolyl ring of L
and M is a divalent lanthanide metal ion selected from Eu, Sm and Yb. p is preferably 2.
The light emitting device of the invention may be a flat panel display.
A number of the complexes of formula (I) are believed to be novel and are also provided by the invention.
The term xe2x80x9calkylxe2x80x9d as used herein is intended to cover branched and unbranched C1 to C6 groups and alicyclic compounds for the C3 to C6 groups. The terms xe2x80x9calkenylxe2x80x9d and xe2x80x9calkynylxe2x80x9d are intended to cover branched and unbranched C2-C6 groups which contain one or more unsaturated Cxe2x95x90C bonds or Cxe2x89xa1C bonds, respectively.
The term xe2x80x9carylxe2x80x9d covers C6 to C10 aromatic groups such as phenyl and naphthyl as well as heterocycles such as pyridyl, furyl and thiophenyl. The term xe2x80x9caralkylxe2x80x9d means C1 to C3 alkyl substituted with aryl, such as benzyl.
The term xe2x80x9chalogenxe2x80x9d means fluorine, chlorine, bromine or iodine. Of these, fluorine is the preferred halogen for the complexes of the invention.
M is Eu (europium), Sm (samarium) or Yb (ytterbium) since of all the lanthanide metals these three form the most stable divalent compounds and complexes. The Eu complexes of the invention are particularly preferred on account of their ability to exhibit bright electroluminescence which, as disclosed herein, may range from yellow to orange to blue.
The trispyrazolyl ligands used in the invention have been found to be particularly effective in terms of their sensitising efficiency i.e., in transferring energy to the metal in the excitation step of the light emitting (e.g., electroluminescence) process. They also impart high radiative efficiencies to the complexes by providing few non-radiative pathways for relaxation (i.e., energy loss from the metal in its excited state). Some of these ligands have been reported. Dias et al, Inorg Chem, 1995, 34, 1975 and 1996, 35, 267 and Renn et al Helv Chim Acta (1995), 78, 993 describe the synthesis of the fluorinated trispyrazolyl borate ligands. Julia et al., Organic Preparations and Procedures International, 16, 299, 1984, discloses the preparation of the trispyrazolylmethane-derived ligands. The conventional synthetic routes to these complexes offer further advantages for light emitting devices in that these materials are prepared in rigorously anaerobic and anhydrous conditions and thus water and oxygen are excluded from the device preparation at all stages. This is beneficial, since water is known to quench photoluminescence in molecular species and both oxygen and water have a deleterious effect upon the properties of light emitting devices.
The complexes of the invention are generally called xe2x80x9corganometallicxe2x80x9d herein. However, it will be understood by those skilled in the art that this term is used synonymously with the term xe2x80x9ccoordinationxe2x80x9d. Also, the term xe2x80x9ccomplexxe2x80x9d as used herein covers both neutral species and compounds containing a charged organolanthanide species as part of a complex salt.
The light emitted from the light emitting devices of the invention may be produced by various mechanisms such as electroluminescence, photoluminescence or cathodoluminescence with electroluminescence being preferred.
In the light emitting devices of the invention, the complexes may be provided as a thin film of the complex itself or in the form of a dispersion of the complex in a polymer matrix. When a matrix is used, it is preferably of the type conventionally used in light emitting (e.g., electroluminescent) devices and based on polymer systems such as polyvinylcarbazole (PVK) or polymethylmethacrylate (PMMA) with a hole transporting material such as 2-(4-biphenylyl)-5-(5-tertbutylbenzene)-1,3,4-oxadiazole (BBO). The organolanthanide complexes used in the invention may themselves act as electron transporting materials, separately from or in addition to being a phosphor, in the light emitting devices.
The ligands ZL3 used in the complexes of formula (I) of the present invention in which R4 and/or R2 is xe2x80x94(CX2)nX or optionally substituted orthodihalogenated or orthodiperhalomethylated aryl are particularly advantageous for use in the light emitting device of the invention since they provide complexes with no carbon-hydrogen (Cxe2x80x94H) bonds within 5 xc3x85 (0.5 nm) of the metal centre. Keeping Cxe2x80x94H bonds away from the metal centre reduces the number of non-radiative pathways for relaxation of the complexes and thus increases their light emitting efficiency.
In the complexes of formula (I), R4 and/or R2 is conveniently trifluoromethyl. Also, R4 and R2 may both be methyl. R3 may be hydrogen and R2 trifluoromethyl. According to one particular embodiment of the invention the organometallic complex has the formula I above in which Z is Hxe2x80x94B and at least one of the L groups is a pyrazolyl group having the formula II or III above in which at least one of R2, R3 and R4 is a trifluoromethyl group. Preferably, in each L group one of R2 and R4 is a trifluoromethyl group and the other is selected from C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl, aryl and aralkyl, as defined above, and R3 is H. A specific example of such a complex has the formula 
This specific complex, which exhibits bright blue luminescence under UV irradiation, belongs to a subclass of europium compounds of formula I having at least one trispyrazolylborate ligand containing CF3 and other ring substituents which may have use as electroluminescence phosphors. The natures of the other variable substituent groups on the pyrazolyl rings act to tune the properties of the complex, such as volatility, solubility and hole and electron transporting properties. It is also found that the presence of the trifluoromethyl substitutents has a protecting effect on the Eu ion thus rendering the complexes stable to air and moisture, even on exposure for several weeks.
As an alternative to the above, ZL3 may be: 
The increased conjugation of the fused bi- and poly-cyclic systems can be advantageous for electron transfer within the complex.
The complexes of formula (I) preferably have p equal to 2 and q equal to zero so that they are neutral overall. Neutral complexes are particularly useful since they are relatively readily vaporised and thus are easy to purify and to deposit as a thin film (e.g., in the light emitting device). However, where the complexes used in the invention are positively charged, they will be provided by compounds containing the complex and a counterion. The counterion preferably should not provide non-radiative pathways for relaxation of the metal. Therefore, it is preferred that the counterion should not contain bonds between hydrogen and other atoms, such as carbon-hydrogen bonds. Trifluoromethylsulphonate, CF3SO3xe2x88x92, halide (fluoride, chloride, bromide or iodide), nitrate, NO3xe2x88x92, and perchlorate, CIO4xe2x88x92, are suitable counterions for this purpose.
In yet another embodiment, the present invention provides a process for producing the complex of the invention, as defined in formula (I), which comprises the steps of reacting M2+ ions (i.e., divalent lanthanide ions) with ZL3 anions in solution and separating the complex from the reaction mixture. The process is carried out in a suitable solvent (e.g., an aprotic solvent such as THF) at about or above room temperature up to the boiling point of the solvent for a time sufficient to form a suitable amount of the complex, preferably with stirring. Where the complex is insoluble in the solvent for the reaction, it may be separated from the reaction mixture by filtration and then, where it is neutral, purified by sublimation (preferably at reduced pressure) or crystallisation from another solvent or solvent mixture. The complex may precipitate from the solvent for the reaction either itself or as a mixed salt (e.g., as a metallate with the other ions in the solution). Where the complex is soluble in the solvent for the reaction, it can be separated from any insoluble by-products (if present) by filtration and obtained as a solid by evaporation of the reaction solvent (preferably under reduced pressure). Again, the complex may be purified by sublimation or crystallisation.
To avoid the presence of water molecules in the complex which might provide non-radiative relaxation pathways, the process is preferably carried out under anhydrous conditions. It is also advantageous to exclude oxygen from the reaction mixture by carrying out the process under an inert gas atmosphere e.g., of nitrogen or argon.
The process may be carried out via the following reaction:
pMxe2x80x2ZL3+MA2xe2x86x92(ZL3)pMAq+pMxe2x80x2A
wherein
Mxe2x80x2 is a monovalent metal such as an alkali metal e.g., sodium or potassium, or thallium or silver (as Tl+ and Ag+, respectively)
p,M,Z and L are as defined above,
and q is 2-p and A is a counterion.
In a preferred version of the process, two equivalents of Mxe2x80x2ZL3 are suspended or dissolved in a solvent and are treated with one equivalent of a divalent lanthanide salt. The reaction mixture is stirred for a period of time between one and 100 hours either at room temperature or at a temperature up to the boiling point of the solvent under standard conditions.